This application is a national stage of PCT/EP95/106597 filed Oct. 17, 1998 and based upon German application 197 55 625.6 of Dec. 15, 1997 under the International Convention.
1. Technical Field
The invention relates to a slot nozzle for coating webs of material, particularly paper or cardboard webs with a pigment coating, having a slot-shaped outlet channel extending over the work width and a distribution chamber extending over the work width, to which the outlet channel is connected.
2. State of the Art
For the coating webs of material, slot nozzles extending over the work width are known for applying to the web of material a defined curtain or jet of coating material. In coating devices which work according to the principle of curtain coating (English: curtain coater), the slot nozzle is arranged above the web extending transversely to the travel direction of the web. The coating material exits the nozzle as a free-falling curtain and falls onto the web. Such a slot nozzle of the generic kind is known from a coating device described in EP 0 327 020-B1, which serves for the production of photographic film. A method of producing coated paper, whereby the pigment coating is applied in a free-falling curtain to a paper web, is known from EP 0 517 223-B1.
In so-called free jet nozzle applicators the slot nozzle is formed by two lips, from which a free jet of coating material under pressure is directed against the web orxe2x80x94in indirect applicationxe2x80x94against a roller. The use of a slot nozzle as a free jet nozzle for coating paper webs is described in DE 296 10 773-U.
In the coating of paper or cardboard webs of large width (for instance 10 m) it is required to produce over the work width a very uniform jet or curtain of coating material. This is particularly hard to achieve when small amounts of pigment coating (for instance 10 ml/m2) have to be applied. Pigment coatings are tend to demix if different shearing conditions exist in the flow. At the same time no disturbing deposits of pigment can be permitted in the supply channels and in the nozzle.
Therefore it is the object of the invention to improve a generic slot nozzle so that it is capable to apply on very wide paper or cardboard webs also small amounts of pigment evenly and free of disturbances over the work width, under variable conditions, e.g. variable viscosity or changing application amounts.
The slot nozzle for coating paper or cardboard webs with a pigment coating, comprises a slot-shaped outlet channel extending over the work width, to which the outlet channel is connected. According to the invention, the distribution chamber is connected to at least two supply channels each having a device for adjusting the supplied volume flow of coating material.
The slot nozzle of the invention offers the possibility to influence the flow conditions in the distribution chamber over the work width and the supplied volume flow.
Preferably, the supply channels are arranged on the side opposite to the outlet channel and the connection distance between two supply channels amounts to 100 mm to 1500 mm, preferably 500 mm to 800 mm.
At a distance of 100 mm to 1500 mm between two supply channels, preferably 500 mm to 800 mm, it is insured that the part of the distribution chamber supplied with coating material by a supply channel is not too large, so that during the cross distribution in the chamber no unacceptable flow differences occur.
Designing the connection between a supply channel and the distribution chamber as a diffuser causes a substantial reduction of the flow velocity when entering the distribution chamber. This prevents a detachment of the coating from the walls and thereby the formation of vortices, which would lead to irregular flow conditions.
Diffusers connected at an angle to the distribution chamber make it possible to use long diffusers when the overall dimensions are small, and to connect the same to supply channels coming substantially straight from one side. Since inside the supply channels the coating is transported at high velocity, in the case of sensitive coatings bent segments in the supply channel could result in undesired separation effects in the mixture.
The bordering wall of the diffuser can be connected to the distribution chamber at an angle xcex1 between 0xc2x0 and 90xc2x0, preferably between 30xc2x0 and 60xc2x0, and the opposite bordering wall at an angle xcex2 between 10xc2x0 and 45xc2x0. In each supply channel a hose clamp valve can be integrated for the adjustment of the volume flow. Preferably, one or both longitudinal walls of the nozzle body a row of heating or cooling elements is arranged. The slot nozzle itself can be arranged in a climatic chamber.
The nozzle body can be composed of two parts, each having a surface-ground inner surface, whereby in one part the distribution chamber is worked in as a groove and that during joining in the area above the distribution chamber a flat material is inserted, which keeps the area below the distribution chamber at the desired distance, in order to form the outlet channel.
According to the invention, coating paint is applied to the web in a free- falling curtain. The outlet channel of the slot nozzle can have a slot width of 150 xcexcm to 600 xcexcm, preferably 250 xcexcm to 350 xcexcm. The distribution chamber can have a height measured parallel to the outlet channel of 10 mm to 100 mm, preferably 20 mm to 60 mm.
The outlet channel of the slot nozzle can have a slot width of 700 xcexcm to 1300 xcexcm. The distribution chamber can have a height measured parallel to the outlet channel of 40 mm to 150 mm.